This invention relates to a precision wafer-positioning device using a solid state actuator for determining an exposure position of a wafer in exposure equipment for semiconductor fabrication.
Generally a positioning device in exposure equipment should have a high degree of position accuracy and precision. The precision should correspond to one-fifth of a minimum line width of a semiconductor circuit pattern. Accordingly, a positioning mechanism having a precision greater than 0.1 .mu.m for a horizontal direction (x,y) and a rotation direction (.gamma.) is required for producing DRAM devices (Dynamic Random Access Memory) of more than 64 megabits. Also, the vertical position must be controlled strictly, as the surface topology becomes larger for larger wafer sizes and a depth of focus becomes small due to an increased numerical aperture of projection optics.
In a rigorous application, such as semiconductor exposure processes, the positioning mechanism should have a precision of less than 0.1 .mu.m in a vertical direction (z) and in tilt directions (.alpha., .beta.). These directions are shown in FIG. 1B. In addition to the precision requirement, the stroke requirements are about 200 mm in the horizontal direction (x,y) and a few hundredths .mu.m in the other four directions (z, .gamma., .alpha., .beta.).
A prior art wafer positioning mechanism for an exposure device has a general configuration as shown in FIG. 1 and is capable of motion along six degrees of freedom: i.e., horizontal directions (x,y), a horizontal rotation (.gamma.), vertical direction (z), and tilt directions (.alpha., .beta.). Such a positioning device is disclosed in the Korean Patent Application No. 86-3573 filed with the Korean Patent Office on May 8, 1986 by the Hitach Company of Japan.
With reference to FIG. 1, a tilt device using three actuators 1 is shown on an x-y table 4, 5 and a turntable 3. The prior art tilt device can make an up-and-down motion based on the fact that a wafer holder 6 moves up-and-down when the three actuators 1 have the same displacements and the wafer holder becomes tilted if the three actuators 1 have different respective displacements. If a tilt device is assembled on a turntable 3 and x-y table 4, 5 for horizontal rotation (.gamma.) and horizontal linear motion (x,y), respectively, Abbe's error occurs owing to a height difference between a wafer surface 7 and a laser beam 8 for an interferometer. It also has a disadvantage of taking a relatively long time for larger prior art positioning devices to settle at an exposure position because of their larger moment of inertia.
For reducing the height of a positioning device, there can be provided an open frame structure having a central space in the x-y table 4, 5 for the horizontal (x,y) motion. By installing the tilt device in the central space, the wafer surface 7 is set to be equal to the laser beam height of the interferometer. However, the area of the positioning device becomes larger owing to the central space of the x-y table 4,5. Accordingly, there is a problem of increased cost for maintaining an expensive clean room for semiconductor fabrication because the overall area of the positioning device is enlarged. Also, the wafer prealignment accuracy can deteriorate in such an open frame structure because a longer stroke of motion is required to load a wafer from a wafer carrier to the wafer holder.